How can I make this program accept input 'value' as only a positive number except for the case 1(where i would like to have both negative and positive numbers as input)?

The input 'value' only takes integers to proceed(it keeps asking for input until the user inputs only a number) but i want the the input 'value' to accept only positive numbers for all cases under the switch y except for the case 1(i.e. I want case 1 to accept both positive and negative numbers)

a = input('Enter the num of your choice between 1 to 4 ');
                    while true
                      value = input('Enter any value that you want to convert:','s');
                       disp(' ')
                        if ~isnan(str2double(value))
                            value=str2double(value);
                          break;
                        else
                            disp('  ')
                            disp('Wrong type of value entered. Try again.');
                           disp('HINT:Make sure to Enter input in numbers.')
                           disp('  ')
                        end
                      end

               switch a
                case 1
                R=value*5

               case 2
               R= value * 6

               case 3 
               R = value*7

               case 4
               R = value*8
            otherwise
            end

[KSSV]

Put a condition:

if a<=0
   fprintf('Please input +ve number')
end

Answer 1

[Are Mjaavatten]

if a == 1
    R = value*5;
else
    if value<0
        disp('Value must be positive')
    else
        R = value*(a+4);
    end
end
disp(R)